One such bolus for intra-rumenal use is described in European Patent Specification No. EP 0 097 507 A. In this device, the core is a pellet of compressed particles, at least some of which comprise the active substance. The cross-sectional area of this pellet varies throughout its length and it is encased by a rigid skin of a brittle material, such as a synthetic resin adhesive, which requires the support of the core to maintain its integrity. As described, one or other end face of the device is cut to expose the core prior to administration to a ruminant. The rumen liquor is then said to act on the exposed face of the core to dissolve it and so release the active substance. It is intended that as the core dissolves, that part, and only that part, of the skin which thereby becomes unsupported, will no longer be able to maintain its integrity and so disintegrates. Thus, the device would release the active substance at either an increasing or decreasing rate according to whether the narrower or broader end face respectively is cut prior to administration.
In practice, it has been found difficult to realise a form of this particular known bolus which functions as intended. This may be due to difficulties in finding an optimum substance for the skin which will not become chipped or partially disintegrate in the rumen whilst the skin is still supported by the core. It could also in part be due to the physical characteristics of such compressed powder cores when coated in such a manner.